Replace TorBirdy's sanitizeHeaders() function

Originally created by @segfault on #17221 (Redmine)

TorBirdy sanitizes the “Date” and “Content-Language” headers of outgoing emails, see https://gitweb.torproject.org/torbirdy.git/tree/components/torbirdy.js#n393.

We should assess whether we can replace that function somehow. I assume that this would require patching Thunderbird.

Feature Branch: feature/17219-replace-torbirdy

Edited May 15, 2020 by segfault

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